The present invention relates to apparatus for cooling and/or remoistening of a moving web.
In drying a moving web of material, such as paper, film or other sheet material, it is often desirable that the web be contactlessly supported during the drying operation in order to avoid damage to the web itself or to any ink or coating on one or more of the web surfaces. A conventional arrangement for contactlessly supporting and drying a moving web includes upper and lower sets of air bars extending along a substantially horizontal stretch of the web. Heated air issuing from the air bars floatingly supports the web and expedites web drying. The air bar array is typically inside a dryer housing which can be maintained at a slightly sub-atmospheric pressure by an exhaust blower that draws off the volatiles emanating from the web as a result of the drying of the ink thereon, for example.
It is often necessary to cool and/or remoisten the web after it has been dried. For example, U.S. Pat. No. 5,333,395 discloses a drying apparatus for traveling webs which includes a cooling tunnel directly connected with the dryer, a combustion chamber for combusting solvent which becomes volatile during drying of the web, heat exchangers, etc. U.S. Pat. No. 5,038,495 discloses a cooling device for cooling a web of material exiting a dryer. The cooling device comprises a substantially closed housing with an inlet and an outlet slit for the web of material. The housing includes a feed aperture at the outlet slit side for feeding outside air into the housing, and a discharge aperture at the inlet slit side for discharging air from the housing into the dryer. Air is fed through the housing counterflow to the direction of web travel. A series of nozzles bring the infed air into contact with the web of material.
Once the traveling web exits a dryer, it is often brought into partial wrapping engagement around a rotating roller or xe2x80x9cchill rollxe2x80x9d so that the web can have substantial intimate contact with the cylindrical surface of the roller for heat transfer purposes to rapidly cool the web. A problem that has persisted in connection with such processes is the tendency for a film of air to intrude between the web and the cylindrical surface of the roller, thereby inhibiting effective contact (and thus heat transfer) between them. It is known that a relatively thin xe2x80x9cboundary layerxe2x80x9d of air is picked up by the moving surfaces of the web and the roller and that some of this air becomes trapped in the wedge-shaped space where the web approaches the roller surface. Unless the web is under a relatively high lengthwise tension, or is moving lengthwise at a relatively low speed, the trapped air enters between the roller and the portion of the web that curves around it, forming a film between the roll and the curved web portion. It will be evident that where a web is to be heated or cooled by a roller around which it is partially wrapped, an insulating film of air between the web and the roller will materially reduce the efficiency of the heat transfer. In addition, where the prior drying operation is drying ink or some other coating that has been applied on the web, the air film that is carried with the moving web may result in solvent condensing on the chill roll surface. The result can be condensate marking, streaking, spotting and/or smudging of the printed web. At higher press speeds (dependent upon web tension and chill roll diameter), the accumulation (thickness) of the condensate film increases and may transfer to the printed web, thereby affecting quality and salability of the finished product. The accumulation and thickness of the condensate is associated with the air gap developed between the web and the chill roll surface, and results in the phenomenon of xe2x80x9cweb lift-off,xe2x80x9d a clearance gap between the web proper and the surface of the roll.
After being heated by the dryer and cooled by suitable means such as a chill roll stand, the web has generally lost a significant amount of its moisture. Excessive moisture loss can cause deleterious curling or waviness of the web.
In heatset web offset printing, a printed web is typically heated in an air flotation dryer to about 250xc2x0 F. or higher to remove mineral oil solvents from the printing ink. Approximately 90% of the mineral oil solvents are removed in the dryer and carried away in the dryer ventilation exhaust air. It is impractical to remove all of the mineral oil solvents in the drying process, since this would be detrimental to the quality of the printed product. The approximately 10% residual solvents remaining in the ink and paper are essentially non-volatile at room temperature. These residual solvents become particularly problematic as the web exits the dryer and initially cools; the residual ink solvents exhibit sufficient volatility to give off solvent vapors, which condense in the surrounding ambient air causing visible smoke. The condensed solvents often deposit on subsequent web processing equipment such as the chill rolls and folding equipment, causing problems of image marking and re-softening of the dried ink on the web.
Various devices such as smoke tunnels and smoke hoods have been used to either contain or direct the solvent vapors during the cooling process to minimize these problems. However, such equipment often becomes contaminated with condensed solvent, which may drip on the web, causing severe quality and productivity problems. Cooling zones, which direct tempered air to the web via flotation nozzles in order to promote cooling without solvent vapor condensation, also have been applied but with limited success in totally eliminating vapor smoke. Such zones cannot cool very rapidly and consequently must be very large and expensive in order to be effective.
Cooling by spraying fine water droplets onto the web have found good success in controlling vapor smoke. However, difficulties arose in promoting a majority of the water droplets to contact the web where they evaporate and remove heat from the web effectively. Local air currents and air film (boundary layer) near the web surface prevent fine droplets from reaching the web surface and reduce the effectiveness of cooling. Furthermore, water spray directed near or beyond the edges of the web does not evaporate by contact with the web, but rather may deposit on other surfaces within the enclosure containing the spray devices and subsequently evaporate. Such deposition cools such surfaces and promotes solvent vapors to condense thereon, causing solvent drips as mentioned above.
Printed or coated webs are processed in a variety of widths according to the requirements of the production order. A spray cooling apparatus such as that of the present invention must have the ability to process the maximum design web width of the printing press as well as small widths, such as webs with widths only 25% of the maximum width. Furthermore, the webs with narrower widths may run at various positions within the maximum width of the press line. Conventional methods of handling the different web widths attempted to inactivate those nozzles positioned outside of the active web width. This required a number of automatic systems to set the water flow valves properly. It therefore would be desirable to effectively and efficiently lower the bulk temperature of the web in order to decrease the heat load of the cooling or chill rolls, or even eliminate the chill rolls or other cooling means such as an air-based cooling zone. Lowered web bulk temperature decreases the evaporation rate of the solvent mixture coating the web, thereby reducing the visible vapors evolving from the web. Condensation that normally occurs at the dryer exit and on the cooling rolls is controlled to a minimum, and the product quality of the web is improved in view of the absence of excessive moisture loss from the web.
It also would be desirable to maximize the contact of water with the web for cooling, and to avoid contact of water droplets on internal surfaces of the device and its enclosure.
It would be further desirable to provide an apparatus and method for the evaporative cooling of a web that accommodated webs having a variety of widths without costly and complicated equipment.
The problems of the prior art have been overcome by the present invention, which provides apparatus and a method for the evaporative cooling of a web within a dryer in a compact manner. Excess fluid overspray is controlled, thereby avoiding problems with fluid depositing on internal surfaces of the dryer, which can cause mineral build-up and can result in web breaks or quality defects if fluid drips onto the web. More specifically, a water delivery system is used which directs a water mist to the web by appropriate design and placement of the spray nozzles. The quantity of water delivered to the web is regulated, and is preferably based upon web temperature. Excess mist is controlled by directing hot air to effectively vaporize the excess mist before it is able to contact the internal surfaces of the dryer enclosure and its internal components.